Nuke Control Rod Cracking Found

Tuesday, July 5, 2011 @ 04:07 PM gHale

GE Hitachi Nuclear Energy found severe cracking in Marathon control rod blades (CRBs) near the end of their nuclear lifetime limits in an international Boiling Water Reactor (BWR/6).

As a result, the design life of certain Marathon CRBs may be less than thought and GE Hitachi is revising the end-of-life limits, according to a Nuclear Regulatory Commission (NRC) information notice.

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The U.S. plants that may contain D-lattice and S-lattice Marathon CRBs are Browns Ferry-1, -2 and -3; Cooper; Clinton; Dresden-2 and -3; Duane Arnold; Grand Gulf; Fitzpatrick; Hatch-1 and -2; Monticello; Nine Mile Point-1; Oyster Creek; Peach Bottom-2 and -3; Perry; Pilgrim; River Bend; Quad Cities-1 and -2, and Vermont Yankee.

In August 2010, GE Hitachi, as part of its surveillance program to monitor Marathon CRB performance, visually inspected four discharged CRBs at an international BWR/6 and found cracks on all four CRBs, according to the NRC notice. The cracks were much more numerous and had more material distortion than those observed in previous inspections of Marathon CRBs. The cracks were also more severe because they resulted in missing boron-carbide capsule tube fragments from two of the inspected CRBs. Additionally, these cracks occurred at locations of lower reported local boron-10 depletion than previously documented.

GE Hitachi attributed the cracking to irradiation-assisted stress-corrosion cracking which results when a material that is susceptible is in an aggressive environment from oxidizing BWR water and experiences excessive stress because of boron-carbide swelling, according to the NRC notice. GE Hitachi determined a significant contributor to the extensive cracking was a rapid thermal transient that occurred when the automatic depressurization system actuated and injected cold water.

GE Hitachi does not anticipate the same severe extent of Marathon CRB cracking in other plants unless a similar or more extreme thermal transient should occur, according to the NRC notice. GE Hitachi also said the cracking phenomenon is only in D-lattice and S-lattice BWR plants. No inspections to date have identified cracks on C-lattice Marathon CRBs. GE Hitachi attributed this to a difference in the C-lattice design that better accommodates boron-carbide swelling.

The GE Hitachi recommended Marathon CRB lifetime reduction imposes a 60-percent local boron-10 depletion lifetime limit, or 54-percent for D-lattice ¼ segment and 55-percent for S-lattice ¼ segment lifetime limit.

GE Hitachi recommends all plants containing D-lattice and S-lattice Marathon CRBs remove the blades from service before they exceed the revised lifetime limits. For any Marathon CRB that exceeds the revised lifetime limit while in use, GE Hitachi advises reactor operation can continue, but the licensee should monitor reactor coolant boron and tritium concentrations in accordance with normal plant procedures, according to the NRC notice. If there is no significant increase in these concentrations, GE Hitachi recommends reactor operation can continue until the end of the cycle before removing the CRBs from service. GE Hitachi recommends licensees get in contact if they detect a significant increase in boron or tritium concentration or both.

GE Hitachi Marathon CRBs consist of a series of absorber tubes, each containing capsules filled with boron carbide (a neutron absorber), welded together to form the control rod blades. Absorber tube cracks allow water to enter the outer absorber tube and the boron-carbide capsules, whereupon boron carbide may leach into the reactor coolant, which could result in reduced control rod worth.

Licensees must maintain control rods in an operable condition as required by plant technical specifications, according to the NRC notice. Technical specification requirements for reactivity anomalies give a limit for the difference between predicted versus measured core reactivity during power operation to ensure a reactivity anomaly, such as change in control rod worth, does not result in a loss of shutdown margin or exceeding specified acceptable fuel design limits. In addition, GE Hitachi advised plant operators to continue monitoring for reactor coolant boron and tritium concentrations in accordance with normal plant procedures to detect CRB cracking.

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